Advanced control strategies for chlorine dioxide generating processes

Abstract

Chlorine dioxide generating processes of the single vessel type which produce chlorine dioxide of high purity are monitored and controlled by a computer using Advanced Control Strategies for steady, stable operation with optimum chemical usage on the basis of a desired chlorine dioxide production rate as the sole input from an operator to the computer program effecting the computer control.

1. A continuous process for the generation of chlorine dioxide at a target production rate, which comprises:
 reducing chlorate ions in an aqueous acid reaction medium in a reaction zone using a reducing agent selected from the group consisting of methanol and hydrogen peroxide and sulfuric acid at the boiling point of the reaction medium under a subatmospheric pressure,  
 removing a gaseous admixture comprising water vapour and chlorine dioxide from the reaction medium,  
 absorbing said gaseous admixture in chilled water in an absorption zone to provide a product aqueous solution of chlorine dioxide,  
 removing a slurry of spent reaction medium and by-product crystalline sulfate from the reaction zone,  
 separating the crystalline sulfate as a by-product from spent reaction medium,  
 adding make-up quantities of chlorate ions, reducing agent and sulfuric acid to the spent reaction medium to form a make-up feed,  
 evaporating water inputted to the process from all sources using steam fed to a reboiler,  
 recycling the make-up feed to the reaction zone, and  
 computer controlling said process on the basis of a desired target chlorine dioxide production rate as the sole input from an operator to a computer program effecting such computer control.  
 
     
     
       2. The process of  claim 1  wherein said chlorate ions are provided by sodium chlorate. 
     
     
       3. The process of  claim 2  wherein said computer controlling operation comprises:
 continuously monitoring the target production rate of aqueous chlorine dioxide solution for changes therein,  
 continuously monitoring the flow rates of sodium chlorate, reducing agent, sulfuric acid, reboiler steam and chilled water to the process, and  
 modifying the initial set points of all said flows in accordance with the changed target production rate.  
 
     
     
       4. The process of  claim 3  wherein said computer controlling operation comprises:
 continuously monitoring the production rate of aqueous chlorine dioxide solution for deviations from the target production rate, and  
 modifying the reducing agent flow rate to maintain the production rate at its target.  
 
     
     
       5. The process of  claim 1  wherein the maximum allowable chlorine dioxide product solution strength and maximum allowable temperature are advised. 
     
     
       6. The process of  claim 4  including:
 continuously monitoring the specification of all material feeds, and  
 modifying the appropriate flow rate set points of said feeds to the reaction zone based on the target production rate and in response to changes in material specification.  
 
     
     
       7. The process of  claim 4  including:
 continuously monitoring sodium chlorate solution physical properties, temperature and density, and, on this basis,  
 creating an on-line virtual chlorate solution analyzer that determines the volumetric concentration of the sodium chlorate solution.  
 
     
     
       8. The process of  claim 7  wherein said on-line virtual chlorate solution analyzer provides an accuracy of about ±0.3% in the sodium chlorate concentration range of about 450 to about 750 gpL. 
     
     
       9. The process of  claim 4  including:
 continuously monitoring the mass input of sodium chlorate to the reaction medium,  
 continuously monitoring the mass consumption of sodium chlorate by the process, and  
 modifying the flow of sodium chlorate to the reaction medium to correspond to the mass consumption of sodium chlorate so as to maintain the sodium chlorate concentration in the reaction medium substantially constant.  
 
     
     
       10. The process of  claim 4  including:
 establishing the boiling temperature set point of the reaction medium based on the expected reaction medium composition,  
 continuously monitoring the temperature of the aqueous acid reaction medium,  
 continuously controlling the temperature of the reaction medium in order to maintain a constant acid normality in the reaction medium, and  
 continuously predicting the acid normality of the aqueous acid reaction medium from the temperature and the chlorate molarity of the aqueous solution.  
 
     
     
       11. The process of  claim 10 , including:
 continuously determining whether the temperature of the aqueous reaction medium differs from the temperature set point, and  
 correcting such deviation by suitable modification to the acid flow rate to the aqueous reaction medium.  
 
     
     
       12. The process of  claim 4  including:
 continuously controlling sodium chlorate molarity in the aqueous reaction medium on the basis of continuously determined system mass balance and adaptive yield tracking.  
 
     
     
       13. The process of  claim 4  including:
 periodically laboratory testing the concentration of sodium chlorate in the reaction medium and monitoring the results of such laboratory testing for a trend in alteration of the concentration of the sodium chlorate in the reaction medium,  
 determining whether or not the concentration of sodium chlorate in the reaction medium has changed in the same direction in a predetermined number of said periodic laboratory tests,  
 in the event, such a change has taken place and provided that the operator has selected the “ADAPTIVE YIELD” function switch, initiating a yield calculation using a series of laboratory tests to determine the applicable adaptive yield.  
 
     
     
       14. The process of  claim 4  including:
 periodically laboratory testing the concentration of sodium chlorate in the reaction medium,  
 determining whether or not the concentration of sodium chlorate in the reaction medium has changed from a target value, and  
 in the event such a change has taken place and provided the operator has selected the “LAB TEST” function switch, applying a one-time bias to the flow rate of sodium chlorate to the reaction medium for a predetermined time to adjust the sodium chlorate concentration in the reaction medium to the target value.  
 
     
     
       15. The process of  claim 4  including:
 maintaining the level of reaction medium in the reaction zone substantially constant by continuously balancing the volume of water flowing to the process and the volume of water evaporated from the reaction medium.  
 
     
     
       16. The process of  claim 4  including continuously determining and displaying the acid normality of the reaction medium. 
     
     
       17. The process of  claim 4  including continuously determining and displaying the concentration of sodium chlorate in the reaction medium. 
     
     
       18. The process of  claim 2  wherein said reducing agent is methanol. 
     
     
       19. The process of  claim 18  including:
 continuously monitoring the production rate of aqueous chlorine dioxide solution, and  
 modifying the feed rate of methanol to the reaction medium in response to fluctuations within a predetermined range based on the initial methanol flow set point.  
 
     
     
       20. A continuous process for the generation of chlorine dioxide at a predetermined production rate, which comprises:
 reducing sodium chlorate in an aqueous reaction medium in a reaction zone using methanol and sulfuric acid at the boiling point of the reaction medium under a subatmospheric pressure,  
 removing a gaseous admixture comprising water vapour and chlorine dioxide from the reaction medium,  
 absorbing said gaseous admixture in chilled water in absorption zone to provide a product aqueous solution of chlorine dioxide,  
 removing a slurry of spent reaction medium and by-product crystalline sodium sulfate from the reaction zone,  
 separating the crystalline sodium sulfate as a by-product from spent reaction medium,  
 adding make-up quantities of sodium chlorate, methanol and sulfuric acid to the spent reaction medium to form a make-up feed,  
 evaporating water inputted to the process from all sources using steam fed to a reboiler,  
 recycling the make-up feed to the reaction zone, and  
 computer controlling said process to produce chlorine dioxide from the reactants with optimum chemical usage on the basis of a desired chlorine dioxide production rate as the sole input to a computer program effecting such computer control.  
 
     
     
       21. The process of  claim 20  wherein said computer program monitors parameters of the process, including:
 aqueous chlorine dioxide solution production rate  
 pressure of the reaction zone  
 temperature, liquid level and sodium chlorate concentration of the reaction medium  
 flow rate of chilled water to the chlorine dioxide absorption step  
 flow rate of aqueous sodium chlorate solution, sulfuric acid and aqueous methanol to the reaction medium  
 flow rate of steam to the reboiler  
 flow rate of make-up water to the process  
 density and temperature of aqueous sodium chlorate feed  
 density of aqueous methanol feed said computer program further generating modification to flow controllers control of the flow rate of  
 chilled water to the chlorine dioxide absorption step  
 aqueous sodium chlorate, sulfuric acid and aqueous methanol to the reaction medium  
 steam to the reboiler.  
 
     
     
       22. The process of  claim 21  wherein said computer program continuously monitors production rate of aqueous chlorine dioxide solution and compares the monitored production rate to the target production rate until a deviation resulting from fluctuations in the process is detected, whereupon the computer program initiates changes in the flow rate of methanol to restore the production rate to the target value. 
     
     
       23. The process of  claim 22  wherein acid flow set point is determined by:
 determining current sulfuric acid consumption, sulfuric acid concentration and target chlorine dioxide production rate  
 calculating the new acid flow set point.  
 
     
     
       24. The process of  claim 22  or  23  wherein methanol dilution water set point is determined by:
 determining current methanol consumption, methanol density and target chlorine dioxide production rate  
 calculating the new methanol flow set point  
 calculating the new methanol dilution water flow set point (SP 40 ).  
 
     
     
       25. The process of  claim 1  wherein chilled water to chlorine dioxide absorption step set point is determined by:
 determining current chlorine dioxide solution strength set point, reboiler steam flow, chilled water flow to chlorine dioxide storage tank vent scrubber and target chlorine dioxide production rate  
 calculating the total required water flow at the target chlorine dioxide production rate  
 calculating condensate flow from an indirect contact cooler for said gaseous admixture to the absorption tower  
 calculating a new chilled water to absorption tower flow set point (SP 70 )  
 calculating the minimum required chilled water flow to the absorption tower (SPMN)  
 determining if SP 70 >SPMN  
 if SP 70  does not exceed SPMN, then the new chilled water flow set point (SP 70 ) is SPMN  
 if SP 70  exceeds SPMN, then the new chilled water flow set point (SP 70 ) is SP 70 .  
 
     
     
       26. The process of  claim 25  including:
 determining if the chlorine dioxide strength or scrubber flow set points have changed  
 if not, effecting chlorine dioxide solution strength control.  
 
     
     
       27. The process of  claim 22  wherein aqueous sodium chlorate solution feed control to the reaction medium is determined by:
 determining current sodium chlorate solution flow, density and temperature, chlorine dioxide flow rate to storage and chlorine dioxide solution strength  
 calculating average chlorine dioxide production rate  
 calculating sodium chlorate usage based on actual chlorine dioxide production rate and yield  
 calculating sodium chlorate concentration in sodium chlorate feed solution  
 calculating required sodium chlorate solution flow rate  
 determining current sodium chlorate molarity and percent solids target, reaction medium level and laboratory test data with respect to sodium chlorate molarity and percent solids in the reaction medium  
 determining if the laboratory test data has changed  
 in the event, that the laboratory test data has changed, determining whether an adaptive yield mode or lab test mode is selected 
 (A)—in the event the lab test mode is selected, calculating the reaction medium volume at the actual operating conditions,  
 
 calculating the aqueous sodium chlorate mass inventory in the reaction medium at actual operating conditions,  
 calculating reaction medium reference liquid volume at reference conditions  
 calculating reference aqueous sodium chlorate mass inventory at reference conditions  
 calculating the difference between said sodium chlorate mass inventories  
 calculating an aqueous sodium chlorate solution flow adjustment bias and applying the bias to the calculated aqueous sodium chlorate flow rate for a predetermined time  
 calculating the aqueous sodium chlorate solution flow set point incorporating the bias for the predetermined time 
 (B)—in the event the adaptive yield mode is selected and in the event, the number of laboratory tests criterion has been met, calculating the chlorine dioxide mass output beginning from the first valid laboratory test of said number of laboratory tests  
 
 calculating the total sodium chlorate mass input beginning from the first valid laboratory test,  
 calculating a corrected sodium chlorate consumption using the last valid laboratory test of said number of laboratory tests in relation to the predicted sodium chlorate concentration in the reaction medium  
 calculating the sodium chlorate adaptive yield correction factor  
 calculating the aqueous sodium chlorate flow set point incorporating the adaptive yield correction factor.  
 
     
     
       28. The process of  claim 22  wherein aqueous reaction medium acidity control is effected by:
 determining the current sulfuric acid flow, aqueous reaction medium temperature, reaction medium target acid normality and sodium chlorate molarity and reaction zone pressure  
 calculating from the latter information, the new sulfuric acid flow set point (SP 20 ) and said flow controller set point limits  
 calculating the reaction medium temperature controller set point (T 20 )  
 calculating the deviation of reaction medium temperature from set point  
 in the event the deviation exceeds a predetermined value, monitoring the deviation and, at the time the reaction medium temperature approaches the set point, resetting the current acid flow set point to the previously calculated value (SP 20 ).  
 
     
     
       29. The process of  claim 22  wherein chlorine dioxide production rate feedback control is effected by:
 determining the deviation (DEV) of chlorine dioxide production rate from the target  
 in the event the deviation exceeds a predetermined value, calculating an adjusted methanol dilution water set point (SP 4 ) from the deviation (DEV) and the initial methanol dilution set point (SP 40 )  
 determining if 
         SP4   -   SP40     SP4       
 
 
       is less than a predetermined value, in which case the adjusted methanol dilution water set point (SP 4 ) is employed, if not, limiting the methanol dilution water set point (SP 4 ) increase to a predetermined percentage of the initial methanol dilution water flow set point (SP 40 ). 
     
     
       30. The process of  claim 22  wherein reboiler steam flow set point determination is effected by:
 determining the target chlorine dioxide production, methanol flow rate, methanol consumption and sulfuric acid density  
 calculating the water load from sodium chlorate, sulfuric acid and methanol dilution water  
 calculating water load from a salt cake filter used to effect separation of crystalline sodium sulfate from spent reaction medium  
 calculating the water load generated in the chemical reaction  
 adding water load from pump purges and seals  
 calculating the reboiler steam flow set point for the total water load from all sources (SP 50 )  
 calculating the minimum reboiler steam flow (FMIN)  
 determining if SP 50 <FMIN, in which case SP 50 =FMIN  
 in the event SP 50  is not less than FMIN, calculating the reboiler steam flow set point deviation and if the deviation exceeds a predetermined value, the SP 50 =SP 50 .  
 
     
     
       31. The process of  claim 22  wherein the reaction medium liquid level control is effected by:
 determining the current reaction medium liquid level, methanol flow rate, methanol consumption, make-up water valve position and position set point  
 calculating the projected chlorine dioxide production rate  
 calculating the minimum (FMIN) and maximum (FMAX) reboiler steam flows  
 determining whether the make-up water valve is open  
 in the event the valve is closed, calculating a steam bias based on reaction medium liquid level deviation  
 in the event the valve is open, determining if the make-up water valve position is greater than the predetermined make-up water valve position set point and, if so, calculating a steam bias based on make-up water valve position deviation  
 calculating the reboiler steam set point incorporating the bias adjustment (SP 5 )  
 calculating change in bias  
 calculating average of change of bias (AVD)  
 determining if AVD is less than a predetermined value and, if so, calculating the average of reboiler steam flow bias  
 calculating the new steam set point (SP 50 ) incorporating the average steam flow bias  
 determining if SP 5 >FMIN and, if not, then SP 5 =FMIN  
 if SP 5 >FMIN, determining if SP 5 <FMAX, if so, then SP 5 =SP 5  and, if not, SP 5 =FMAX.  
 
     
     
       32. The process of  claim 22 , wherein the maximum allowable chlorine dioxide solution strength and temperature interlock is determined by:
 determining the current reaction zone pressure, chlorine dioxide solution temperature, chlorine dioxide solution strength set point, process air implied valve position and maximum allowable chlorine dioxide solution strength by storage design (MS)  
 determining if the process air valve is open and if so, then the maximum chlorine dioxide solution strength and temperature interlock set points take their predetermined values  
 in the event the process air valve is close, converting the chlorine dioxide solution temperature to Kelvins  
 calculating the water vapour pressure above the chlorine dioxide solution  
 calculating the chlorine dioxide partial pressure above the solution and correcting for air leakage  
 calculating Henry's constant for chlorine dioxide at the operating solution temperature  
 calculating the new maximum chlorine dioxide solution strength (SPM) at its partial pressure and temperature  
 determining if SPM<MS and if not SPM=MS and if so, displaying and entering the new maximum allowable chlorine dioxide solution strength interlock set point and SPM=SPM  
 calculating Henry's constant for the chlorine dioxide solution from its strength set point and chlorine dioxide partial pressure  
 converting the chlorine dioxide solution temperature from Kelvins  
 displaying and entering the maximum allowable solution temperature interlock set point (PTM).